Fall 2005

September 21, 2005
Adri van Duin
Materials and Process Simulation Center
Beckman Institute (139-74)
Caltech

Title: Development and applications of ReaxFF, a transferable computational method for atomistic-scale dynamical simulations of chemical reactions

This lecture will provide an overview of the applications and background of the ReaxFF reactive force field method. ReaxFF is a bond-order dependent force field method that includes a geometry- dependent polarizable charge distribution, allowing reactive, atomistic scale dynamic simulations at a computational expense magnitudes lower than quantum mechanical (QM) simulations. While initially developed for first-row elements1, over the last years we found the method to be highly transferable, allowing us to develop ReaxFF descriptions for covalent, metallic, ionic and mixed systems all across the periodic system.

Ongoing projects involving ReaxFF include:

• Reactions on Ni/Pt/Ru anodes and cathodes in fuel cells
• Hydrogen and oxygen transfer through fuel cell electrolytes
• Carbon deposition and nanotube growth catalyzed by transition metals
• Hydrogen storage in magnesium and magnesium fluorides
• Stress-induced cracking in silicon and aluminium oxides
• Temperature and chemistry-induced failure of silicon polymers
• Nitramine and peroxide-based high-energy material sensitivity
• Energy release of nitramine/metal/metal oxide composite materials
• Enzyme catalysis
• Iron melting at the temperatures and pressures of the Earth's core
• Dielectric breakdown in silicon-based semiconductors

This lecture will address the highlights from this research and will discuss the concepts behind ReaxFF and its relation to other computational simulation methods, including its implementation in a multiscale simulation environment (CMDF) that allows coupling of different length- and time scales.



September 28, 2005
Prof. Joel Moore
Dept. of Physics
UC Berkeley

Title: The spin drag and spin Hall effects in semiconductors: theory and experiment

Spin transport in semiconductors is fundamentally different in several ways from charge transport, with implications for both fundamental science and advanced electronics. This talk covers two new phenomena in semiconductor transport that have been studied intensively over the past few years. We first discuss how "spin Coulomb drag" tends to reduce spin diffusion strongly relative to charge diffusion, even when both spin and charge are carried by the same particles. This effect has recently been confirmed quantitatively in optical experiments. We then review the current understanding of the "spin Hall effect" in semiconductors, and how subtle differences between materials are very important for the stability of this effect. We discuss in particular recent work on how a Z2 topological index for noninteracting band structures determines the stability of the spin Hall effect even when interactions and disorder are present.



October 5, 2005
Dr. Andrij Baumketner
UC Santa Barbara

Abstract



October 12, 2005
Dr. Vasyl Yuchyshyn
Big Bear Solar Observatory

Title: How Solar Magnetic Fields Determine Severity of Geomagnetic Storms

I will present results of several studies dedicated to solar magnetic fields, coronal mass ejections (CME) and magnetic clouds (MC). We are elaborating an approach to routinely forecast magnitude of the B_z component in an interplanetary ejecta and the intensity of geomagnetic storms 1-2 days in advance by analyzing solar magnetic fields and measuring speeds of halo CMEs as they propagate across the LASCO C3 field of view.

Our approach is based on the following results. Recently we found that the B_z component in the interplanetary magnetic field (IMF) is correlated with the projected speed of CMEs. The relationship is better pronounced for very fast ejecta with speeds higher than 1200 km/s, while slower events display larger scatter. In turn, the B_z in the IMF is correlated with the intensity of the Dst index of geomagnetic activity.

From point of view of space weather, only the most intense storms (Dst < -200 nT) possesses a certain danger to humans. Therefore, we selected and analyzed magnetic configurations only of those solar structures, which produced CMEs, associated with these intense events. It seems that magnetic fields of the source regions of these CMEs can be separated in 4 distinct classes: i) delta sunspots; ii) "tadpole"-shaped sunspots; iii) magnetic complexes and iv) quiescent filaments. I will discuss details of the structure of the magnetic field of these active regions and also how this information can be helpful in solar physics research and space weather forecasts.



October 26, 2005
Prof. Deborah Fygenson
Dept. of Physics
UC Santa Barbara

Title: DNA Nanotubes: Design and Control

Short sequences of DNA can be designed to self-assemble into extended structures based on Watson-Crick pairing rules. The most generic design scheme makes use of building blocks, or "tiles", of three or more strands that hybridize into a core of cross-linked double helices with single-stranded sticky ends. We use tiles known as double-crossovers (DX units) to self-assemble tubular polymers of DNA that are up to 100 µm in length, ~10 nm in diameter and correspondingly stiff. We can explain and manipulate these characteristics via sequence design and assembly conditions. I will describe recent results aimed at elucidating the thermodynamics of DNA nanotube nucleation and growth that will lay a foundation for practical applications of this and other tile-based nucleic acid architectures.



Past Department Colloquia

September 7, 2005
Dr. Michael Corbin
Dept. of Astronomy
Arizona State University



September 14, 2005
Prof. Quyen Nguyen
Department of Chemistry
UC Santa Barbara

Title: Tuning Intermolecular Attraction to Create Polar Order and One-Dimensional Nanostructures

Recently there has been significant interest in utilizing functional organic semiconductor materials for electronic and opto-electronic devices. However, better materials and further understanding of their electronic properties are critical for devices based on these materials. In this work, we use various techniques to study the molecular interactions in self-assembled thin films of hexa-substituted aromatics. Because of the crowding in the aromatic rings, each subunit has a dipole moment parallel to the stacking direction that sum as they stack. Depending on the amide side-chains, the molecules can either form long wires parallel to the surface or columnar stacks perpendicular to the substrate. The distance between monomers within the columnar structure can be tuned by different chemical functional groups. The assembly of the wires can be directed with electric fields. Scanning tunneling microscopy studies show different molecular packing and spacing between monomers as the side chains are altered. With proper tuning chemical structures, the self-assembly of hexa-substituted aromatics into columnar structures could be used to study electronic transport properties in one-dimensional organic semiconductors.